In the manufacture of printed circuit cards and boards, a dielectric sheet material is employed as the substrate. A conductive circuit pattern is provided on one or both of the major surfaces of the substrate.
In order to plate on the substrate, it must be seeded or catalyzed prior to the deposition of metal thereon. Included among the various dielectric materials suggested for such purpose are various organic polymers including polyimides.
Among the more widely employed procedures for catalyzing a substrate is the use of a stannous chloride sensitizing solution and a palladium chloride activator to form a layer of metallic palladium particles thereon. For instance, one method of catalyzing a dielectric substrate is exemplified by U.S. Pat. No. 3,011,920 which includes sensitizing a substrate by first treating it with a solution of a colloidal metal, activating the treatment with a selective solvent to remove unreactive regions from the colloids on the sensitized dielectric substrate, and then electrolessly depositing a metal coating on the sensitized substrate, for example, with copper from a solution of a copper salt and a reducing agent.
Also, as suggested, for example, in U.S. Pat. No. 3,009,608, a dielectric substrate can be pretreated by depositing a thin film of a "conductivator" type of metal particle, such as palladium metal, from a semicolloidal solution onto the dielectric substrate to provide a conducting base which permits electroplating with conductive metal on the conductivated base.
In addition, there have been various suggestions of treating substrates with certain materials in order to enhance the attachment to the substrate of a non-noble metal catalyst. For instance, U.S. Pat. No. 4,301,190 suggests a pre-wet treatment of a substrate with an "absorption modifier" to enhance the attachment to the substrate of a non-noble metal catalyst. Certain surfactants, hydrous oxide sols and certain complexing agents are suggested as "absorption modifiers".
However, the methods of catalyzing, or seeding, various organic polymer substrates and particularly polyimide substrates, have not been entirely satisfactory and improvement in the degree of adhesion of the final metal layer to the substrate has been less than desired.
In the packaging of semiconductor chips, polyimide films are often coated onto substrates. For instance, in the formation of multilayer substrates for mounting chips, one configuration employs an insulating substrate of a ceramic material onto which is deposited a pattern of metallic conductors. Usually, the conductors are three layers of metal being a layer of chromium, followed by a layer of copper, followed by an overlying layer of chromium. On top of the metallized ceramic substrate is placed a layer or film of a polyimide, and on top of the polyimide a second layer of a pattern of conductors is provided.
The metal contacting the polyimide on the second or subsequent layers can be, for instance, copper as disclosed in U.S. Pat. No. 4,386,116 to Nair, et al. and assigned to International Business Machines Corporation, the assignee of the present application, disclosure of which is incorporated herein by reference.
However, the adhesion between the copper and polyimide is not entirely satisfactory and could stand a degree of improvement. In particular, problems have occurred at the copper-polyimide interface resulting in lifting up of the copper line from the underlying polyimide substrate rendering the carrier unsuitable for its intended purpose.
Accordingly, in order that the configurations which involve a copper-polyimide interface be competitive in a commercial environment, it is necessary to improve the adhesion at the copper-polyimide interface.
Another use would be as a dielectric and/or circuit carrier for flexible circuits. This could involve spray coating or roller coating polyamic acid onto a sheet of metal (such as stainless steel or aluminum). The film is then cured or imidized, resulting in a film which is fully or substantially fully cured. The metal which the polyimide is on can be imaged, removed, or maintained. On top of the polyimide, three layers of metal are deposited such as by either evaporation or sputtering. The conductors are chromium or nickel, followed by a layer of copper, followed by a layer of chromium or nickel. By means of photolithographic operations, this metal is imaged into circuits. Depending on the use of the circuit, the cured polyimide may or may not be imaged, either before or after the formation of the circuit.
Flexible circuits may also be fabricated using free-standing polyimide films onto which metal layers are vacuum deposited, laminated, or glued. The metal circuit pattern is defined by using a photoresist pattern to either act as a plating mask or act as a mask for subtractive etching of the metal layer. Through-holes in the polyimide film can be made by drilling, punching, or etching.
In addition, the selective etching of fully imidized polyimide films to provide openings or vias therein is important for various uses of polyimide. For instance, in the packaging of semiconductor chips, polyamic acid films are often coated onto substrates and then cured either chemically or thermally.
In a number of these situations, it is necessary to form vias in the polyimide layer to allow for electrical connections to be made between the different layers of metallurgy. In order that the interconnection be as accurate as possible, it is necessary that the polyimide films be fully cured to avoid distortion of the desired polyimide pattern and prevent attack from other wet processing chemicals.
For instance, in the formation of multi-layer substrates for mounting chips it is necessary to electrically contact some of the conductors in the upper or second layer of metallization to some of the conductors on the lower or first layer of metallization. In order to do so, the polyimide must be selectively etched to form the desired vias therein to allow for metal connection between the upper and lower levels of metallization and connection to a chip and/or board.
Wet etching fully of substantially fully cured polyimide may be accomplished using either hydrazine hydrate, ethylenediamine, or concentrated caustic solutions. These are dangerous chemicals and are avoided by industry wherever possible. Ethylenediamine is highly toxic and irritating and hydrazine hydrate causes blindness and is extremely explosive. Concentrated solutions of sodium or potassium hydroxide are highly corrosive and can cause severe burns. If used, all three methods would require tools which are extremely expensive because of safety concerns.
It would, therefore, be desirable to provide a process for etching of fully cured or substantially fully cured polyimide that is relatively fast without requiring dangerous or explosive chemicals.